Fair Value Measurements - Long-term Debt (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Carrying Value	$ 2,243.5	$ 2,234.5
Senior Loans [Member]
Debt Instrument [Line Items]
Carrying Value	364.0	382.4
Junior Loans [Member]
Debt Instrument [Line Items]
Carrying Value	26.8	31.1
Fixed Rate Loan [Member]
Debt Instrument [Line Items]
Carrying Value	672.7	625.1
Equipment Notes, Series A 2015-1 [Member]
Debt Instrument [Line Items]
Carrying Value	363.6	378.6
Equipment Notes, Series B 2015-1 [Member]
Debt Instrument [Line Items]
Carrying Value	76.0	80.0
Equipment Notes, Series C 2015-1 [Member]
Debt Instrument [Line Items]
Carrying Value	103.8	109.5
Equipment Notes, Series AA 2017-1 [Member]
Debt Instrument [Line Items]
Carrying Value	235.4	242.5
Equipment Notes, Series A 2017-1 [Member]
Debt Instrument [Line Items]
Carrying Value	78.5	80.8
Equipment Notes, Series B 2017-1 [Member]
Debt Instrument [Line Items]
Carrying Value	77.2	83.7
Equipment Notes, Series C 2017-1 [Member]
Debt Instrument [Line Items]
Carrying Value	85.5	85.5
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Carrying Value	160.0	135.3
Estimated Fair Value [Member]
Debt Instrument [Line Items]
Estimated Fair Value	2,322.0	2,169.6
Estimated Fair Value [Member] | Level 3 [Member] | Senior Loans [Member]
Debt Instrument [Line Items]
Estimated Fair Value	382.4	373.6
Estimated Fair Value [Member] | Level 3 [Member] | Junior Loans [Member]
Debt Instrument [Line Items]
Estimated Fair Value	27.4	31.1
Estimated Fair Value [Member] | Level 3 [Member] | Fixed Rate Loan [Member]
Debt Instrument [Line Items]
Estimated Fair Value	700.4	600.1
Estimated Fair Value [Member] | Level 3 [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Estimated Fair Value	160.0	135.3
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series A 2015-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	382.7	374.8
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series B 2015-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	78.2	78.1
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series C 2015-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	105.9	107.9
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series AA 2017-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	240.7	228.8
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series A 2017-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	78.9	76.6
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series B 2017-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	77.8	79.1
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series C 2017-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	$ 87.6	$ 84.2